Multiple object tracking (MOT) experiments have suggested that the capacity limit for attention is approximately four objects (Pylyshyn and Storm, 1988; Yantis, 1992; Cowan 2001; Scholl and Xu, 2001). However, accuracy in these paradigms has ranged from 65% to over 90% for four objects. This variation may be due to differences in the MOT paradigms used, which may have affected the difficulty of the task.

Subjects performed a basic MOT task, tracking five out of ten balls. Subject performance was analyzed as speed, size, or total number of objects was varied. This allowed us to explore the manner in which each affected task difficulty and the concurrent effect that had on the capacity limit of attention.

As both the speed of the balls and the number of distractors increased, performance decreased. Performance as ball size varied showed an inverted U shape, with peak performance at a ball size of 1.5°. Performance for both smaller (0.5° or less) and larger balls (2.75°), was significantly worse.

These results demonstrate that increasing the task difficult, by either increasing the speed of the objects, number of distractors, or by using smaller or larger objects, will have a significant effect on an individual's performance on the MOT task. These data support the view that capacity limit for the number of attended objects is not simply a fixed number, but instead varies with the attentional demands of the objects.